您可以使用此类型：

type A  = {
        prop1: string
        prop2: B
    }
    
   type B = {
        prop3: string
        prop4: boolean
    }
    
    type UnionToIntersection<U> = (U extends any ? (k: U) => void : never) extends ((k: infer I) => void) ? I : never;
    type Flatten<T, K extends keyof T> = UnionToIntersection<T[K]> & Omit<T, K>;
    
    type Both = Flatten<A,'prop2'>
    
    const result: Both = {prop1: 'one', prop3: 'two', prop4: false}

编辑：这更简单：

type Flatten<T, K extends keyof T> =  Omit<T, K> & T[K];
type Both = Flatten<A,'prop2'>